This invention relates to a protective coating for thermal paper, and more particularly to a protective coating for images formed on the paper and which provides protection from exposure to light, and provides resistance to solvents, soiling and abrasion.
In the field of product labeling, it is common practice to print information such as weight, price, and contents on a product label in bar code form so that it may be read by a scanning machine. The information is generally printed by conventional ink printing apparatus or by thermal printing means. The use of these product labels in every market is increasing. However, especially where thermal paper is used, the printed images on the labels tend to fade after exposure to light and are susceptible to exposure from various elements in the environment such as oils, alcohols, and other solvents during shipment and storage. As a result, it becomes difficult for a scanning machine to read the printed matter on the label accurately.
Many attempts have been made to provide a protective coating for machine-readable labels which will protect the printed image from exposure to solvents. Quinn et al, U.S. Pat. Nos. 4,600,630 and 4,670,295, disclose a protective overcoating for a machine-readable marking. The composition comprises a liquid oligomer and liquid monomer comprised of acrylics which are curable by ultraviolet light. However, this coating is applied only as an overcoat to the marking and is applied only on glass or nonporous materials.
Arbree et al, U.S. Pat. No. 4,591,887, relates to a solvent-resistant thermally printable material for the manufacture of labels comprising a protective layer of a polymeric resin on top of a thermally imprintable color producing layer and an adhesive layer.
Marinelli et al, U.S. Pat. No. 4,740,495 discloses a protective coating for thermal paper labels comprising a thermally reactive layer, and a top coat containing fluorochemical material for solvent protection.
Although Arbree and Marinelli et al disclose protection of thermal images on paper from solvents, neither provides protection to the label from exposure to fluorescent light or sunlight. Further, both Arbree and Marinelli involve multilayer coatings requiring a more complex coating process.
Doi, U.S. Pat. No. 4,886,774 discloses a protective overcoating for thermal paper which protects images against fading from exposure to ultraviolet radiation. However, the Doi composition requires the addition of a separate ultraviolet radiation absorbing composition to provide resistance to fading.
Accordingly, there is still a need in the art for a simple protective coating composition for thermal paper which is effective, easy to apply, and provides protection to thermal images from solvents and abrasion as well as protection from fading by exposure to light.